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Loving Hut in Archway (but you probably know it and it's obviously not straight-up Chinese).
The restaurant in Egerton Mews in Knightsbridge used to be 100% vegan Chinese until about 15 years ago. They then changed into an omnivorous other Asian (I can't remember which Asian cuisine exactly) place, but you could still order from the old vegan menu. As I haven't been for ages, I don't know if they've kept this up, but if they have it might be London's best-kept vegan secret by now (oops).
I can't remember which one it was, but it might have been at number 8 (in the basement) and may now be called Yaowarat Thai Restaurant. It might be worth giving them a call, but rather than being a well-kept secret, this info might also be well out of date by now. :)
Anywhere in particular?